Fatal error: Exception thrown without a stack frame in Unknown on line 0

Trying to upgrade from 5.5.2.1 to 5.6.0.1, I received a white browser window with the following message:

Fatal error: Exception thrown without a stack frame in Unknown on line 0

No chance to login again. PHP memory is 64 MB. I have a lot of add-ons in /packages, and blocks in /blocks, maybe one of them caused this error, I don’t know.

After deleting all that stuff from /updates, /files/cache, /files/tmp, /files/trash, the error message has disappeared, but there is still the white window and I can’t log in.

I’ll do a fresh install of 5.5.2.1 and then try to upgrade to 5.6.0.1, and then will see how it behaves (on the same host).

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Okay, here’s my experiment:
I created a fresh installation of 5.5.2.1 on the same host, upgraded from the dashboard to 5.6.0.1, and this time it works.
So, there must be something strange in my old installation that confused the upgrade. If I find out, I will tell you.

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Same error while upgrading a customer’s web site, but this time a hint appeared: